The contents of Japanese Patent Application No. 11-372877, with a filing date of Dec. 28, 1999, in Japan, is incorporated by reference herein, in its entirety.
The present invention generally relates to a canister structure preferably used for an internal combustion engine of a motor vehicle.
An internal combustion engine that combusts gasoline as a fuel usually has a canister to reduce the amount of evaporative fuel (or HC) released into the atmosphere.
The canister generally has a charge portion through which the evaporative fuel is introduced into the canister and a purge portion through which a vacuum pressure is applied into the canister, and these are disposed at a first end portion of the canister. The canister also has a drain portion at a second end portion thereof, through which the canister is open to the atmosphere.
The canister adsorbs the evaporative fuel introduced through the charge portion by means of adsorbent material disposed therein. In a predetermined engine operating condition, the atmospheric air is introduced into the canister through the drain portion, and adsorbed HC is thereby desorbed from the adsorbent material. In this manner, the atmospheric air purges the adsorbed HC off the adsorbent material. The evaporative fuel purged from the canister is sucked into an air intake system of the internal combustion engine through the purge portion, accompanied with the atmospheric air, and is combusted in the internal combustion engine.
Since the adsorption/desorption of the evaporative fuel is an exothermic/endothermic reaction respectively, the temperature of the adsorbent material increases when it adsorbs the evaporative fuel and the higher temperature tends to lower the adsorbence (i.e. the capability of adsorbing) of the adsorbent material. Conversely, the temperature of the adsorbent material decreases when it desorbs the adsorbed HC and the lower temperature tends to lower the desorbence (i.e. the capability of desorbing) of the adsorbent material.
Considering these characteristics of the adsorbent material, it is known, in the prior art, to lesser the decrease in the adsorbence and the desorbence by increasing the specific heat of the adsorbent material for preventing the temperature thereof from changing as much as possible. Japanese Provisional Patent (Kokai) Publication No. 10-339218 (1998) discloses a kind of adsorbent material that is made of activated charcoal bound together with heat-storing particles having high thermal conductivity and high heat capacity uniformly on the surface of the activated charcoal in order to increase the specific heat of the adsorbent material as a whole.
In the prior art described above, the adsorbent material cannot desorb all the adsorbed HC, and some amount of it remains adsorbed because the desorbence of the adsorbent material decreases as mentioned above, and further because of aging of the adsorbent material in the canister, or when the internal combustion engine is in a specific operating condition. Especially when the adsorbed HC remains near the drain portion, it migrates in the direction to the drain portion, forced by other evaporative fuel that is newly introduced from the charge portion. The remaining adsorbed HC that migrates to the drain portion may drain out of the canister, so that the amount of evaporative fuel released into the atmosphere may increase. This detracts from the most important characteristic of the canister, that is, reducing the amount of HC released into the atmosphere.
However, since the remaining HC adsorbed on any other part of the canister can be re-adsorbed downstream closer to the drain portion, when it migrates by newly introduced evaporative fuel, it causes few problems. Therefore, the amount of evaporative fuel released into the atmosphere by migration largely depends on the density of adsorbed HC (i.e. the amount of HC adsorbed in per unit amount of the adsorbent material) near the drain portion after a purge process has finished.
Some structural modifications to decrease the density of adsorbed HC mentioned above are considered, such as enlarging the capacity of the canister, or increasing the specific heat of the entire adsorbent material by binding heat-storing particles together uniformly on the surface thereof. However, these structures increase the desorbence of the adsorbent material even in parts of the canister other than near the drain portion, where it has no effect on the ability to reduce the amount of evaporated fuel released into the atmosphere. In considering the restriction concerning the size of the canister that is able to be equipped on the motor vehicle, such increase in size relatively restricts the improvement of the desorbence near the drain portion. Thus the evaporative fuel released into the atmosphere due to the migration cannot be reduced fully effectively although it makes more difficult to arrange the canister in the motor vehicle because of the increase in its size and also because of the corresponding increase in the cost of the canister.
Therefore, a general object of the invention is to provide an improved canister structure that alleviates one or more of the shortcomings discussed earlier herein.
An object of the invention is to provide a canister structure for an internal combustion engine that can provide an improved ability to reduce the amount of evaporated fuel released into the atmosphere, while also minimizing any increase in size and cost.
The above and other objects of the present invention can be accomplished by a canister structure for an internal combustion engine that includes a container having a gas passage therein through which gas containing evaporative fuel flows from a first end to a second end, a purge portion disposed on the container and connected to the first end through which vacuum pressure is applied into the gas passage, a charge portion disposed on the container and connected to the first end through which the gas is introduced into the gas passage, a drain portion disposed on the container and connected to the second end through which the gas passage is open to the atmosphere, and adsorbent material, disposed in the gas passage, to temporarily adsorb the evaporative fuel, whose specific heat on the side of the second end is made higher relative to that on the side of the first end.
It is essential that at least the adsorbent material disposed on the side of the second end has a specific heat higher than that on the side of the first end, thus the adsorbent material may be divided into a first adsorbent material disposed only on a purge and charge side of the gas passage and a second adsorbent material, which has a higher specific heat relative to the first adsorbent material, disposed only on a drain side of the gas passage.
In the preferred embodiment of the invention, the adsorbent material having a higher specific heat is made by mixing heat-storing material or activated charcoal having higher specific heat with the normal adsorbent material mainly made of activated charcoal.
According to one aspect of the present invention, since the adsorbent material having a higher specific heat disposed on the side of the drain portion prevents the temperature of the adsorbent material from changing due to the desorption of the adsorbed HC as much as possible, the canister can improve the desorbence of the adsorbent material, and reduce the amount (or the density) of the adsorbed HC remaining on the adsorbent material after purging. Therefore, the canister of the present invention can reduce the evaporated fuel released into the atmosphere due to the migration of the adsorbed HC.
Furthermore, when the second adsorbent material is disposed only on the side of the canister open to the atmosphere, and not disposed elsewhere, the necessary amount of the heat-storing material or the activated charcoal with a higher specific heat is lessened. Therefore, the canister can achieve the desired result, while the size and cost of the canister is kept as small as possible. The small size makes it easier to arrange the canister in the motor vehicle.
Generally speaking, as long as the same volume of adsorbent material is disposed, the smaller the sectional area of the gas passage is, the longer the length of the gas passage is made although the loss of pressure through the gas passage increases.
Therefore, it is preferable that a sectional area of the gas passage on the side of the second end is made smaller than that on the side of the first end. In this manner, the canister of the present invention can improve the desorbence of the adsorbent material near the drain portion, and reduce more effectively the amount of evaporated fuel released into the atmosphere by the migration.
As a way of narrowing the gas passage near the drain portion, the gas passage in the container may have a U-shape configuration. With the structure, the container can be made in a simple shape, such as a cylindrical or a box shape.
More particularly in this case mentioned above, it is preferable that the canister further includes a partition wall that divides the container into a larger chamber and a smaller chamber, where the smaller chamber is disposed on the side of the drain portion.
The canister of the present invention may further include a heat-insulating layer disposed in the gas passage, and the adsorbent material may be divided into a first adsorbent material disposed on the purge and charge side of the gas passage and a second adsorbent material, which has a higher specific heat relative to the first adsorbent material, disposed on the drain side of the gas passage. In this structure, the heat-insulating layer will be interposed between the first and second adsorbent material.
Through this structure, the heat-insulating layer can protect the second adsorbent material from being affected by the change of temperature of the first adsorbent material due to the desorption of the adsorbed HC. This reduces the temperature change of the second adsorbent material so that the desorbence of the second adsorbent material can be improved.
In the preferred embodiment of the invention, the heat-insulating layer is a heat-insulating filter made of foamed ceramic.
Further objects, features and advantages of the present invention will become apparent from the Detailed Description of Preferred Embodiments which follows when read in light of the accompanying figures and the claims.